METHOD OF INTERFERENCE-TOLERANT TRANSMISSION OF HEARTBEAT SIGNALS

Abstract: The invention relates to a method of interference-tolerant heartbeat measurement, in which method a person's heartbeat signal is measured at a suitable part of the body and transmitted by telemetric transmission from a transmitter to a receiver as pulse message formed by successive measuring pulses, and in which a transmission interval of at least some pulse signals is proportional to the person's measured heartbeat rate. According to the invention, each measuring pulse is formed of at least two identification pulses (C1, C2), whereby the time interval (tx) between the identification pulses corresponds to a specific time interval determined for each transmitter-receiver pair, on the basis of which specific interval the receiver identifies the pulse signals intended for itself.

Description: Met h od of interference - tolerant transmission of heartbeat signals This invention relates to a method of inter¬ ference-tolerant heartbeat measurement, in which method a person's heartbeat signal is measured at a suitable part of the body and transmitted by telemet- ric transmission from a transmitter to a receiver as pulse data formed by successive measuring pulses, in which data a transmission interval of at least some pulse signals is proportional to the person's measur¬ ed heartbeat rate. Known telemetric transmitters of heartbeat mea¬ suring devices typically transmit a burst of about 5 kHz each time they detect an ECG signal. A transmit- ter circuit is constituted by a simple resonance cir¬ cuit activated by transistor control. At heartbeat measurement, a transmitter unit sends a signal each time the heart beats. A receiver calculates the heartbeat rate on the basis of the difference of time between the successive signals transmitted, which means that the method in principle consists of time interval coding, i.e. that the data to be transmitted is included in the transmission and coded into the time between the pulses. The known method described above is simple and reliable in conditions free of interference. Single interferences can be filtered off by comparing the obtained pulse value with preceding results: if a new measurement result differs too much from the preced- ing ones, it is obviously caused by some outer inter¬ ference coupled to the transmission-reception channel and may thus be eliminated from the measurement re¬ sults. However, if wireless transmission of pulse data is performed in an interfered environment, the situa- tion changes essentially. A continuous irregular pulse train may then arrive at the receiver, from which train to pick up the correct pulse signal is a difficult task, often even impossible. Such a situa- tion arises easily when two or several users of pulsimeters are too close to each other. The object of this invention is to provide methods by means of which the above-mentioned draw¬ backs are avoided and a coding of a pulse signal is effected in such a way that it is possible to pick up the correct signals from an incoming interfered sig¬ nal flow reliably and accurately without deteriorat¬ ing the original accuracy of the moment of timing the signal. To achieve this, the method according to the invention is characterized in that each transmission of pulse data comprises at least two identification pulses, whereby the time interval between the identi¬ fication pulses corresponds to a specific time inter- val determined for each transmitter-receiver pair, on the basis of which specific interval the receiver identifies the measuring pulses intended for itself. By means of the method of the invention, it is possible to pick up the correct pulse data from the incoming interfered signal flow reliably and accura¬ tely without deteriorating the original accuracy of the moment of timing the signal carrying the pulse measuring information. Preferred embodiments of the invention are characterized in what is disclosed below in the at¬ tached claims. In the following the invention will be describ¬ ed in greater detail by means of embodiments with reference to the attached drawings, in which Figure 1 shows a block diagram of a known tele- metric heartbeat measuring device, to which the meth¬ od according to the invention can be applied, Figure 2 illustrates schematically a burst sig¬ nal applied to magnetic coils of a transmitter ac- cording to Figure 1, Figure 3 illustrates an embodiment of the meth¬ od of coding pulse data according to the invention, Figure 4 illustrates another embodiment of the method of coding pulse data according to the inven- tion, Figure 5 illustrates a third embodiment of the method of coding pulse data according to the inven¬ tion, Figure 6 illustrates the principle of a fourth embodiment of the invention. Electrodes (not shown) of a telemetric heart¬ beat measuring device shown in Figure 1 are connected to differential input terminals of an ECG preamplifi¬ er. A pulse signal given by the preamplifier 1 is amplified in an AGC amplifier 2, by which a power amplifier 3 is controlled, which generates an ac sig¬ nal according to Figure 2, i.e. a burst signal, con¬ trolling coils 4. A magnetic field detected by re¬ ceiver coils 5 is amplified in a sensitive preampli- fier 6, and then the signal is applied to a signal amplifier 7. An output signal of the signal amplifier is processed in a microcomputer 8, which stores the pulse data calculated at the measuring stage in a memory 9 and displays them on a liquid crystal dis- play 10. In practice, transmitter electronics transmits a burst of about 5 ms. The receiver repeats the burst substantially equal in duration. However, the receiv¬ er circuit requires after the burst a recovery time, during which it is not capable of separating a new transmission burst from the previous one. Experiments have shown that this time varies between 10 and 20 ms. On account of this, the present pulsimeters can¬ not use a pulse interval shorter than 20 ms, and thus, such an interval cannot be included in the cod¬ ing according to the invention. A coding of identification and pulse data can start from a basic period of about 1 ms, correspond¬ ing to the shortest detectable difference of ah iden- tification code based on time interval. To this coded data is added 25 ms, so that the shortest possible pulse interval will be sufficiently separated from a break pulse period. If necessary, devices requiring shorter recove- ry times may be developed, whereby the time available and thus the number of identification codes may be increased. Hereafter, the word message signifies a group of pulses, containing separate or combined identifi- cation pulses and heartbeat data pulses. Accordingly, message pulse is a common name for all pulses in¬ cluded in a transmission. One embodiment of the method of the invention appears from Figure 3, in which a coding of pulse data into a time interval t.^ is implemented by using a double pulse, whereby a person's measured heartbeat rate is proportional to the transmission interval t x of double pulse signals. In a coding method of Figure 3, two message pulses Cl and C2 separated by a fixed time interval t x are used instead of a single measur¬ ing pulse. The fixed time interval t x between these message pulses Cl, C2 is interpreted, according to the invention, as an identification code of said transmission and the time interval between two such message pairs as the pulse data to be transmitted. By means of the procedure according to this embodiment, it is possible to separate almost all interfering messages from the correct signal. However, if an interfering signal arrives at the receiver at a moment which is at a distance of the used code from the correct message pair, before or after,, it is impossible to pick up the correct message from a pulse train having equal intervals. Such a coverage situation is usual when simultaneous transmissions of two transmitters close to each other mix with each other. The signals overlap each other at certain intervals, depending on the mutual differ¬ ence between the time intervals of the message pulses, i.e. the codes of the pulse data. The closer these time intervals are to each other, the more sel¬ dom they cover each other - but the longer at a time. On the other hand, if the difference between the time intervals is great, the message pulses coincide pro¬ bably only at one of the successive transmissions, but overlaps succeed each other more often. Due to the overlap phenomena, it is, however, impossible to separate transmissions completely reli¬ ably by using double pulse technique, but this tech¬ nique is applicable to measuring devices intended for amateurs, for instance, whereby a less critical view can be taken of the correctness of each meter read¬ ing, when the meter is used in the vicinity of other meters. Figure 4 illustrates the principle of another embodiment of the invention, using triple pulse cod¬ ing for separating pulse messages from each other. At triple pulse coding, one break pulse is added with respect to the method of Figure 2 before actual mes¬ sage pulses transmitting an identification code. A break pulse A is transmitted during a fixed, very short (20 ms) time interval t 2 before identification pulses Cl and C2. In the receiver, the break pulse is at first searched for: initially, each incoming pulse is as- sumed to be a break pulse. If the following transmis¬ sion is received during a correct time interval (20 ms), the latter identification pulse is expected next. If the next pulse arrives at the right time, the pulse data is transmitted further to a calcula- tion logic for determining the pulse rate value. If interferences mix into the transmission channel, they can in this case arrive at the receiver either before the break pulse, between the break pulse and the first identification pulse, or between the identification pulses. An interference arriving before the break pulse A causes a problematic situation only if it arrives just at the break time (20 ms) before the break pulse. In this case two pieces of information in length of the break pulse interval arrive at the identification logic. The correct message is then separated by means of the two identification pulses: an identification sequence started upon a first in¬ terfering pulse is interrupted by an error situation at the first identification pulse. A new correct se¬ quence can then be started upon the genuine break pulse - this presupposes naturally that the arriving pulses are stored for a later change of the starting moment of the sequence. An interfering pulse occurring between the break pulse A and the first identification pulse Cl may also interfere an already started identification sequence. The time interval and timing of the two original break pulses may, however, be restored by adding together the two time intervals shorter than the known break pulse interval. An interference occurring between the identifi¬ cation pulses Cl and C2 changes the code of the mes¬ sage pulses into a wrong code. A wrongly timed pulse coming at this stage can be ignored, and concentra¬ tion can be focussed on expecting the next, correct identification pulse C2. If such a pulse comes at the right moment, the receiver operates normally. Other¬ wise, the transmission is interpreted as erroneous. As a variation of the coding according to Fig¬ ure 4, the pulse data may be formed according to Fig¬ ure 5 by transmitting at first the first and the sec¬ ond identification pulse Cl and C2, the time interval t x of which still corresponds to the specific time interval of the transmitter-receiver. Not until then a break pulse A is transmitted after a fixed time interval t 2 . In this case, the identification pulses are at first searched for in the receiver. The trans¬ mission intervals of the identification pulses may be measured e.g. continuously, irrespective of whether the pulse data is intended for said receiver or not. When the time interval between the identification pulses indicates that the pulse data is intended for said receiver, the break pulse is expected, and if it is received after the correct time interval t 2 (20 ms), the pulse data is transmitted further to the calculation logic for determining the pulse rate value. The examination of interference situations set forth in connection with the embodiment of Figure 4 is applicable to the solution of Figure 5. Nearly all interference situations which are due to a simultaneous use of two transmitters having different identification data may be derived from the above-described situations caused by one interfering pulse. However, there is a small theoretical possibi- lity that a single interfering pulse is switched on a transmission having a longer interval between the identification pulses in such a way that the entity also may be interpreted as a transmission containing shorter identification. This is also true vice versa, which means that, after a shorter identification field, one interference arriving exactly at the right moment may cause the same identification problem. The situation is, however, very unlikely and, according- ly, it does not restrict the use of the coding method according to the invention for a reliable pulse mea¬ surement also under demanding conditions. On the basis of the above, it has been empiri¬ cally found out that a completely reliable transmis- sion and reception of simultaneous pulse data on two or several channels require: - transmission of a break pulse A; - transmission of two identification pulses Cl and C2; and - storage of at least 6 successive pulse inter¬ vals in a buffer of the receiver for separating si¬ multaneous transmissions. The method according to the invention may also be easily applied to coding pulse data into the mes- sage pulses. In that case the pulse data to be trans¬ mitted, coded into a pulse interval, is inserted ac¬ cording to Figure 6 between a break pulse A and iden¬ tification pulses Cl, C2 e.g. in such a way that, subsequent to the break pulse A, a first measuring pulse D of the pulse data is transmitted after a fixed time interval t 2 and then, after a time interval t 3 proportional to the pulse rate data, a second mea¬ suring pulse is transmitted, which simultaneously is the first identification pulse Cl. Subsequently, a second identification pulse C2 is transmitted, the time interval t x between these identification pulses being equal to the specific time interval determined for said transmitter-receiver, as described earlier. Since the pulse data in this case is included in the message pulses, the time between successive transmissions does not necessarily contain any infor¬ mation. To minimize the probability of interference situations caused by overlap phenomenon, the trans¬ mission interval of the message pulses can, according to one embodiment of the invention, be set to be ran¬ domly variable. Experiments have shown that a working transmission interval is an interval in which the time between transmissions ( j in Figure 3) varies randomly between 2 and 4 seconds. Random time contri- butes considerably to the purpose of separating sev¬ eral transmissions faultlessly. On the other hand, the transmission can be made safer according to another embodiment of the inven¬ tion by including the pulse data both in the message pulses and in the time between the transmissions, through which a transmission coding as reliable and tolerant of interference as possible is provided. According to still one embodiment of the inven¬ tion, the message pulses may contain e.g. a consecu- tive number or some other identification, on the basis of which the receiver may discover the missing or extra transmissions. By using a configuration of a four-pulse message according to Figure 6, the time interval t 3 between the pulse after the break pulse and the subsequent pulse may also be used for this purpose. In the following, reference is made to Fig¬ ure 6 by using the same reference numerals. To begin with, a break pulse A is transmitted, as described above. Then after a fixed time interval, a first identification pulse D is transmitted, and subse- quently, after the time interval t 3 proportional to the consecutive number or the like of the transmis¬ sion, a second identification pulse Cl is transmit¬ ted. After that a third identification pulse C2 is transmitted, the time interval between the second and third identification pulses being equal to the speci¬ fic time interval determined for said transmitter- receiver. The specific time interval of a transmitter- receiver may be fixed or it may preferably also be formed when starting the transmitter by a random operation, to which the receiver is locked for the time of the measuring procedure. The starting takes place e.g. by taking the receiver close to the trans- mitter, due to which the high level of a received signal starts the measurement, for instance in the way presented in the Finnish Patent 88972 of the ap¬ plicant. It is clear to one skilled in the art that the different embodiments of the invention are not re¬ stricted to the examples shown, but that they can vary within the scope of the claims below. The scope of the invention thus also allows a use of pulse transmissions which are more complicated than the ex- amples shown and combine, for instance, the functions of two or several embodiments described here. How¬ ever, the only essential thing is that the time in¬ terval between the two identification pulses contain¬ ed in the transmission corresponds to the specific time interval determined for each transmitter- receiver.

Claims: Claims : 1. Method of interference-tolerant heartbeat measurement, in which method a person's heartbeat signal is measured at a suitable part of the body and transmitted by telemetric transmission from a trans¬ mitter (1 to 4) to a receiver (5 to 10) as pulse data formed by successive measuring pulses, in which data a transmission interval of at least some pulse sig- nals is proportional to the person's measured heart¬ beat rate, c h a r a c t e r i z e d in that each transmission of pulse data comprises at least two identification pulses (Cl, C2), whereby the time in¬ terval (t x ) between the identification pulses corre- sponds to a specific time interval determined for each transmitter-receiver pair, on the basis of which specific interval the receiver identifies the measur¬ ing pulses intended for itself. 2. Method according to claim 1, c h a r a c - t e r i z e d in that the pulse data is formed by sending, at intervals proportional to the person's measured heartbeat rate, pulse signals formed by a break pulse (A), subsequent to which the first iden¬ tification pulse (Cl) is sent after a fixed time in- terval (t 2 ) and the second identification pulse (C2) after the specific time interval (t x ) of the transmit¬ ter-receiver. 3. Method according to claim 1, c h a r a c ¬ t e r i z e d in that the pulse data is formed by sending, at intervals proportional to the person's measured heartbeat rate, pulse signals formed by the first and the second identification pulse (Cl, C2), the time interval (t x ) between the pulses correspond¬ ing to the specific time interval of the transmitter- receiver pair, subsequent to which a break pulse (A) is sent after a fixed time interval (t 2 ). 4. Method according to claim 1, c h a r a c ¬ t e r i z e d in that the pulse data is formed by sending a break pulse (A), subsequent to which a first measuring pulse (D) is sent after a fixed time interval (t 2 ) and then after a time interval (t 3 ) pro¬ portional to the pulse data a second measuring pulse (Cl) is sent, which simultaneously is the first iden¬ tification pulse, and after that the second i ' denti- fication pulse (C2), the time interval (t x ) between the identification pulses (Cl, C2) being equal to the specific time interval determined for said transmit¬ ter-receiver pair. 5. Method according to claim 4, c h a r a c - t e r i z e d in that the transmission interval of the pulse data is set to be randomly variable in such a way that the time between the pulse data varies randomly within predetermined limits, preferably be¬ tween 2 and 4 seconds. 6. Method according to claim 4, c h a r a c ¬ t e r i z e d in that the transmission of the pulse data is doubled by sending, besides the pulse data (t 3 ) included in the measuring pulses (D, Cl), also the same data coded into a time interval (t^ between the pulse data. 7. Method according to claim 1, c h a r a c ¬ t e r i z e d in that the pulse data is formed by sending, at intervals proportional to the person's measured heartbeat rate, pulse signals formed by a break pulse (A), a first identification pulse (D) sent after a fixed time interval (t 2 ), a second iden¬ tification pulse (Cl) sent subsequently after a time interval (t 3 ) proportional to a consecutive number or the like of the pulse data and a third identification pulse (C2) sent after that, the time interval (t x ) between the second and third identification pulses being equal to the specific time interval determined for said transmitter-receiver pair. 8. Method according to any of the foregoing claims, c h a r a c t e r i z e d in that the spe¬ cific time interval (t x ) of the transmitter-receiver pair is formed by a random operation when the trans¬ mitter is started, to which time interval the receiv¬ er is locked for the time of the measuring procedure.

Inventors: HEIKKILAE ILKKA, PIETILAE ARTO, HEIKKILAE, ILKKA, PIETILAE, ARTO
Applicants: HEIKKILAE ILKKA, PIETILAE ARTO, POLAR ELECTRO OY, HEIKKILAE, ILKKA, PIETILAE, ARTO
Application year: 1994
Main classification: G01D5
IPC codes: A61B5, G08C19